The sensible place to build new houses, factories and offices is where people are, in cities and towns where infrastructure is in place.

where people are为融合型关系副词where引导的名词性关系分句，相当于一个名词词组。where集先行词与关系副词于一身，等于the place where，其中the place为先行词，where为关系副词引导定语从句修饰the place. 所以，where people are = the place where people are. 相当于被关系副词where引导的定语修饰的地点名词词组。where在从句中作表语（方位副词可以做表语）。正因为where people are本质上是一个名词词组，故可以进一步后接定语。in cities and towns where...为介词短语，修饰where people are 中隐含的名词place，为定语。

译文：建造新住房、新工厂、新办公室的理想位置就是城镇中人多、基础设施齐全的地方。

in place：就位、就绪。